Low heat loss and secure chip carrier for cryogenic cooling

ABSTRACT

An integrated circuit package is disclosed. The integrated circuit package includes a substrate having a cavity formed therein for enclosing an integrated circuit. The integrated circuit package also includes a carrier for holding the integrated circuit. The carrier is positioned within the cavity of the substrate. A thermally reactive connector is coupled to the carrier. The thermally reactive connector is for selectively coupling the carrier to the substrate when a temperature of the thermally reactive connector is above a first temperature such that the carrier is held in position within the opening. The thermally reactive connector is also for decoupling the carrier from the substrate when the carrier temperature is at the first temperature. In this manner, the number of thermal paths between the integrated circuit and the substrate of the integrated circuit is reduced.

This is a divisional of application Ser. No. 08/309,056, filed Sep. 20,1994.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an integrated circuit (IC)package for enclosing an IC and, more particularly, to an IC packagehaving a secure chip carrier for cryogenic cooling of the IC chip.

2. Art Background

Techniques for using cryogenic coolants such as liquid helium forreducing the operating temperature of an integrated circuit (IC) arewell known in the prior art. One method for cryogenic cooling requiresthe use of a cold probe having a length of thermally conductivematerial. The cold probe includes a base end that is connected to acryogenic cooling plant within which a cryogenic coolant is disposed andcirculated. The cold probe also includes a contact end which istypically connected directly to the die of the IC via a tight thermaljoint. As the base end of the cold probe is in direct contact with thecryogenic coolant, the temperatures of the cold probe and the die arereduced to approximately the temperature of the cryogenic coolant.Cryogenic cooling typically enhances the performance of the cooled IC.

FIG. 1 shows a typical prior art IC package design in which an IC 101 isepoxied to the substrate 103 of the IC package 100. Bond wires 102 arecoupled between the IC and conductors disposed in the substrate 103.These conductors lead to the I/O pins 105. Lid 104 covers the cavity inwhich the IC 101 is placed. The flow arrows 106 illustrate thermal pathsfrom the IC 101. The flow arrows 106 are shown to be directed from theIC package 100 and flowing to the ambient air outside of the package100. This happens, for example, when the operating temperature of the IC101 is greater than the ambient temperature of the atmosphere outside ofthe IC package 100. When the operating temperature of the IC 101 isgreater than the ambient temperature of the air, it is desirable to haveas many thermal paths away from the IC 101 as possible such thatsufficient heat transfer may take place.

For cryogenic cooling of the IC 101, the package design of the ICpackage 100 is typically not altered in any substantial way. Forexample, the cold probe is often connected to the top surface of the dieof IC 101, wherein the bottom surface of the IC 101 remains connected tothe substrate 103. Unfortunately, the thermal paths through the ICpackage 100 become a liability during cooling because the direction ofheat transfer reverses, and, unless the thermal load of the IC 101 isisolated from the ambient air, the cryogenic cooling plant will beforced to cool the ambient air.

The prior art discloses that an efficient method for isolating the IC101 from the ambient air is to create a vacuum between the IC 101 andthe ambient air. However, a typical prior art method for creating avacuum requires surrounding the entire IC package 100 with an evacuatedchamber. This solution requires a large amount of board space in acomputer system and may significantly increase the footprint of thesystem. A further disadvantage of the prior art method is that permanentattachment of the cold probe to the die of the IC results in waste ifeither of the cold probe and the IC fails while the other continues tobe operable.

As will be described, the present application discloses an integratedcircuit package having a reduced number of thermal paths to anintegrated circuit during the cryogenic cooling of the integratedcircuit. The integrated circuit package of the present inventionachieves isolation of the integrated circuit by creating a vacuum withinthe integrated circuit package itself.

SUMMARY OF THE INVENTION

An integrated circuit package is disclosed. The integrated circuitpackage includes a substrate having a cavity formed therein forenclosing an integrated circuit. The integrated circuit package alsoincludes a carrier for holding the integrated circuit. The carrier ispositioned within the cavity of the substrate. A thermally reactiveconnector is coupled to the carrier. The thermally reactive connector isfor selectively coupling the carrier to the substrate when a temperatureof the thermally reactive connector is above a first temperature suchthat the carrier is held in position within the opening. The thermallyreactive connector is also for decoupling the carrier from the substratewhen the carrier temperature is at the first temperature. In thismanner, the number of thermal paths between the integrated circuit andthe substrate of the integrated circuit is reduced.

In another embodiment, the integrated circuit package includes aconnector coupled to the carrier. A cooling apparatus is provided fordetachable connection to the connector. The cooling apparatus includesan outer wall that is disengageably attached to the integrated circuitpackage such that an airtight seal is formed between the outer wall andthe integrated circuit package when the outer wall is attached to theintegrated circuit package. The cooling apparatus also includes a coldprobe disposed within the outer wall such that a chamber is formedbetween the outer wall and the cold probe. The cold probe isdisengageably attached to the carrier. The cold probe is for cooling thecarrier to a first temperature when the cold probe is attached to thecarrier. Evacuation of the chamber results in evacuation of the cavity.In this manner, isolation of the integrated circuit is achieved withoutbulky enclosures and the cryogenic cooling apparatus and integratedcircuit are not permanently attached.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, features, and advantages of the method and apparatus of thepresent invention will be apparent from the following detaileddescription of the invention in which:

FIG. 1 shows a prior art integrated circuit package.

FIG. 2A shows an integrated circuit package according to one embodiment.

FIG. 2B shows an integrated circuit package according to anotherembodiment.

FIG. 3A is a top view of an integrated circuit package.

FIG. 3B is a bottom view of an integrated circuit package.

FIG. 4 shows the connection between the carrier and the substrate of anintegrated circuit package.

FIG. 5 shows a cooling apparatus according to one embodiment.

FIG. 6 shows a cooling system according to another embodiment.

FIG. 7 is a flow chart illustrating a method according to oneembodiment.

FIG. 8 shows a flexible circuit.

DETAILED DESCRIPTION

In the description that follows there are disclosed a number of specificdetails and examples. These details and examples, as well as the detailsand examples illustrated in the accompanying figures, are provided toillustrate the operation of the various embodiments of the presentinvention. Accordingly, it will be understood that these details andexamples are not exhaustive of the number of ways in which the presentinvention may be practiced.

FIG. 2A illustrates an integrated circuit package according to oneembodiment. The IC package 200 includes substrate 103, lid 104,thermally reactive connectors 207, carrier 205, connector 210, andchamber interface 215. As shown, the IC 101 is placed on carrier 205,which acts as a platform for supporting the IC 101. Although the ICpackage 200 is shown as receiving the IC 101, the IC package 200 may bemodified to receive a multi-chip module comprised of a number of ICs.Attachment of the IC 101 to the carrier 205 may be accomplished, forexample, by the use of epoxy or any other equivalent bonding agent. Thecarrier 205 is preferably manufactured from a thermally conductivematerial such as copper to provide a minimal thermal resistance betweenthe IC 101 and a cold probe (not shown). Although the carrier 205 isshown as having the same thickness as the thinnest portion of thesubstrate 103, the thickness of the carrier 205 may be reduced tominimize thermal overhead.

Thermally reactive connectors 207 are attached to the carrier 205. Thethermally reactive connectors 207 are preferably manufactured of lengthsof bimetallic material, memory metal, or of any other thermallyconductive material that changes states when a trip-point temperature isreached. The thermally reactive connectors 207 are shown as extendingacross air gaps between the carrier 205 and the substrate 103. When thetemperature of the carrier 205 is above the trip-point temperature forthe thermally reactive connectors 207, flexure in the thermally reactiveconnectors applies force to the substrate 103 such that the carrier 205is held in place. Thus, in an ambient state, the thermally reactiveconnectors 207 act as clips for holding the carrier 205 in place. Whenthe carrier temperature drops to the trip-point temperature, thethermally reactive connectors 207 change state such that substantiallyno force is applied to the substrate 103 and the carrier 205 isdecoupled from the substrate 103 such that the thermal paths between thesubstrate 103 and the IC 101 are limited to those thermal paths createdby the bond wires 102. Thus, in a cooled state, the thermally reactiveconnectors 207 are not in contact with substrate 103. This significantlyreduces the number of thermal paths between the ambient and the IC 101.The trip-point temperature for the thermally reactive connectors 207 ispreferably chosen to be between an ambient temperature and the minimumtemperature achievable by cryogenic cooling.

Bimetallic materials are typically manufactured of two metals havingdifferent rates of thermal expansion. When the bimetallic material isheated or cooled to a trip-point temperature the different rates ofthermal expansion results in tension between the two metals causing thebimetallic element to assume a second shape. In this case, when thetemperature of the carrier is above a trip-point temperature, thebimetallic elements are each compressively connected to the substrate103. But when the carrier temperature drops to the trip-pointtemperature, the bimetallic element curves back away from the substratesuch that the only connection between the substrate 103 and the IC 101are the bond wires 102. Memory metal behaves similarly.

The design of IC package 200 provides for the detachable connection of acold probe (not shown) to the IC package 200. Connector 210 is attachedto carrier 205 and is preferably manufactured of the same thermallyconductive material as carrier 205. Alternatively, the carrier 205 andconnector 210 may be integrally formed of a single piece of thermallyconductive material. For this embodiment, the connector 210 is shown asa truncated cone that tapers as it extends downwardly from bottomsurface of the carrier 205. The cold probe (not shown) is designed toreceive the connector 210 in a male-female connection. As will bediscussed below, the cold probe provides support for the carrier 205when the thermally reactive connectors 207 are detached from thesubstrate 103.

Lid 104 is attached to substrate 103 to provide an air-tight seal forthe cavity formed in the substrate 103. The lid 104 is preferably put inplace after the carrier 205 is clipped to the substrate 103 using thethermally reactive connectors 207 and after the bond wires 102 have beenattached. In this manner, the only manner in which gases may enter andexit the cavity of substrate 103 is via the air gaps formed between thecarrier 205 and the substrate 103. Chamber interface 215 is designed toform an air-tight seal with the outer wall of a cooling apparatus (notshown). The outer wall encloses the cold probe, and an annular chamberis formed between the cold probe and the outer wall. This annularchamber is in communication with the cavity of substrate 103 such thatevacuation of the chamber results in evacuation of the cavity. Thus, thethermal load of IC 101 can be isolated from the ambient without using abulky enclosure to enclose the entire IC package 200.

FIG. 2B shows an alternative IC package 200 that uses a flexible circuit115 in place of the bond wires 102 for connecting the IC 101 to the I/Opins 105. At high frequencies of operation, bond wires 102 manufacturedof gold or a similar conductor can act as inductors, reducing theintegrity of the signals as they are transmitted from the IC 101 to theI/O pins 105. To reduce the inductive effects of the bond wires, theflexible circuit 115 is used. The flexible circuit 115 includes aflexible material such as kapton™ upon which traces of conductivematerial are deposited. The flexible circuit 115 is placed in directcontact with the IC 101, and the length of the path between the IC 101and I/O pins 105 is reduced, which reduces the amount of inductanceassociated with each electrical connection. Wherein the electricalconnection is improved, care must be taken to ensure that the parasiticthermal losses associated with the electrical connection are notunnecessarily increased. Therefore, the amount of flexible materialshould be minimized to reduce the thermal paths between the IC 101 andthe I/O pins 105. The flexible circuit 115 is discussed in more detailwith respect to FIG. 8, below.

FIG. 3A shows a top view of IC package 200. The bond wires 102 are shownas extending from the IC 101, across the air gaps surrounding IC 101 andto the substrate 103. The carrier 205 is shown as being rectangular inshape, but may be any shape sufficient to contact all of the bottomsurface of the IC 101. The substrate 103 has a rectangular hole toprovide for placement of the carrier 205 within the rectangular hole.Thus, the air gap between the carrier 205 and the substrate 103 is alsorectangular. As shown, each thermally reactive connector 207 extendsfrom a corner of he carrier 205, diagonally across the air gap and to aninterior corner of the of the substrate 103 formed by the rectangularhole.

FIG. 3B shows a bottom view of IC package 200. The chamber interface 215is shown as being annular in shape, but any shape of the chamberinterface sufficient to enclose the air gaps formed between thesubstrate 103 and the carrier 205 may be alternatively employed.Connector 210 is clearly shown as having a circular cross-section.Thermally reactive connectors 207 are shown as extending from the bottomof carrier 205 and in positions substantially parallel to correspondingthermally reactive connectors attached to the top of carrier 205. Inthis manner, force is applied between each corresponding top and bottomthermally reactive connectors 207 such that each pair of thermallyreactive connectors 207 acts as a clip.

FIG. 4 more clearly shows the thermally reactive carriers 207. The solidlines for the thermally reactive connectors 207 correspond to theambient state when the thermally reactive connectors act as clips. Thedashed lines correspond to the cooled state when the thermally reactiveconnectors are detached from the substrate 103.

It is important that good thermal connections are provided between thecarrier 205 and the IC 101, between the carrier 205 and the connector210, and between the carrier 205 and the thermally reactive connectors207. A good thermal connection between the carrier 205 and the IC 101ensures proper cooling of the IC 101. Good thermal connections betweenthe carrier 205 and the connector 210 and between the carrier 205 andthermally reactive connectors 207 ensure a more rapid cooling of thecarrier 205 such that the thermally reactive connectors enter the cooledstate more rapidly. Thus, the IC 101 is preferably epoxied to thecarrier 205, the carrier 205 is preferably soldered or integrally formedwith the connector 210, and the thermally reactive connectors 207 arepreferably soldered to the carrier 205.

FIG. 5 is shows a cryogenic cooling apparatus 600 according to oneembodiment. Shown therein is cold probe 500, which is a solidcylindrical piece of thermally conductive material. A portion of thethermally conductive material is removed from the tip of the cold probein order to provide a receptacle 510 for receiving the connector 210 ofthe IC package 200. The interface between the cold probe and theconnector 210 may be any equivalent interconnection type. The cold probe500 and the connector 210 are preferably manufactured of thermallyconductive materials having different coefficients of thermal expansionsuch that a tight thermal joint is created between the connector 210 andthe receptacle 510 when cooling occurs. When the connector 210 andreceptacle 510 are heated to the ambient temperature, the connector 210may be easily detached from the cold probe. Thus, the unnecessary wasteof operative components can be avoided should only the cryogenic coolingapparatus 600 or the IC 101 fail. The operative component may berecycled in another similar cryogenic cooling system. To achieve thisthermally variable joint, the connector 210 may be manufactured of alloy49, and the cold probe 500 may be manufactured of copper.

Disposed around the cold probe 500 is a cylindrical outer wall 515. Anannular chamber 520 is created between the outer wall 515 and the coldprobe 500. The bottom wall 550 seals the bottom of the chamber. A pinchtube 525 is provided for creating a vacuum once the cooling apparatus iscoupled to the IC package 200. The upper portion of the outer wall isprovided with a ledge 530 for receiving the chamber interface 215 (seeFIGS. 2A and 2B) such that an air tight seal is formed when the ICpackage 200 is connected to the cooling apparatus 600. The cooling ofthe cold probe 500 is provided by cryogenic engine (not shown) which isattached to the cold probe via the bottom of the chamber.

FIG. 6 shows a cooling system according to one embodiment. The ICpackage 200 is shown as being connected to cryogenic cooling apparatus600. The chamber interface 215 rests on the ledge 530 such that anair-tight seal is formed between the IC package 200 and the cryogeniccooling apparatus 600. The connector 210 is received within thereceptacle 510 of the cold probe 500. The annular chamber 520 and thecavity of substrate 103 have been evacuated and the pinch tube 525 hassealed the chamber. Thus, a passive vacuum is created. Alternatively, anactive vacuum may be maintained to isolate the thermal load of the IC101.

The temperature of cold probe 500, IC 101, carrier 205 and thermallyreactive connectors 207 is been reduced such that the thermally reactiveconnectors 207 are detached from the substrate 103. The IC 101 is thusisolated from the ambient by a vacuum created within the IC package 207.The number of thermal paths between the substrate 103 and the IC 101 arealso significantly reduced.

FIG. 7 is a flow chart showing a method of one embodiment. At step 700,an IC package as described in FIGS. 2-4 is assembled. At step 710, theIC package is coupled to the cryogenic cooling apparatus described inFIG. 5. At step 720, the chamber and cavity are evacuated to isolate theIC from the ambient air outside of the IC package. The IC is cooled viathe cold probe at step 730 until the temperature of the carrier andthermally reactive connectors is reduced to the trip-point temperatureat step 740.

FIG. 8 shows the flexible circuit 115 of FIG. 2B in greater detail. Theflexible circuit 115 includes a flexible material 805 upon which traces810 of conductive material are deposited. A first end of each trace 810terminates at a center face of flexible circuit 115. A solder bump 815is placed at the first end of each trace 810 to attach the center faceof the flexible circuit 115 to the IC 101 (as shown in FIG. 2B). Asecond end of each trace 810 terminates at the outer perimeter of theflexible circuit 115. A solder bump or similar attachment device isdeposited at the second end of each trace 810 for attachment to bondpads of the substrate 103 (as shown in FIG. 2B). The bond pads of thesubstrate 103 are directly connected to the I/O pins 105 (not shown).The traces 810 thus provide an electrical connection between the IC 101and the I/O pins 105 using a reduced amount of condutive material overthe bond wires 102.

As shown, there are slots cut into the flexible circuit 115 to reducethe total amount of flexible material 805. This reduces the thermalpaths between the IC 101 and the substrate 103. Further, the slots allowthe vacuum to surround the IC 101. Slots and vents in the flexiblematerial may be of any size, shape, or orientation so long as the slotsand vents do not degrade the electrical connection.

In the foregoing specification the invention has been described withreference to specific exemplary embodiments thereof. It will, however,be evident that various modifications and changes may be made theretowithout departing from the broader spirit and scope of the invention asset forth in the appended claims. The specification and drawings are,accordingly, to be regarded in an illustrative rather than restrictivesense.

What is claimed is:
 1. A process for manufacturing a cryogenic coolingsystem for an integrated chip, the process comprising the stepsof:attaching the integrated circuit to a carrier having a thermallyreactive connector attached thereto; positioning the carrier within abottom opening of a substrate of a semiconductor package, thesemiconductor package having a cavity defined by a side wall, the bottomopening, and a top opening, the carrier being positioned such that theintegrated circuit extends into the cavity; coupling the carrier to thesubstrate via the thermally reactive connector when a carriertemperature of the carrier is at an ambient temperature such that thethermally reactive connector holds the carrier in place within theopening of the substrate; attaching at least one bond wire between theintegrated circuit and the carrier; and covering the top opening with alid.
 2. The process of claim 1, wherein the process comprises thefurther steps of:coupling the carrier to a cooling apparatus, thecooling apparatus having an outer wall for enclosing the bottom openingand a cold probe for cooling the carrier, the cold probe being enclosedwithin the outer wall such that a chamber is formed between the outerwall and the cold probe, the chamber being in atmospheric communicationwith the cavity via the bottom opening; evacuating the chamber and thecavity; and cooling the carrier such that the thermally reactiveconnector automatically decouples the carrier from the substrate whenthe carrier temperature reaches a first temperature.